This invention relates to magnetic resonance (MR) imaging systems. More particularly, it relates to a radio frequency (RF) coil for use within an MR imaging system for homogeneous quadrature transmit and multiple channel receive by the RF coil.
A magnetic resonance (MR) imaging system provides an image of a patient or other object in an imaging volume base on detected radio frequency (RF) signals from precessing nuclear magnetic moments. A main magnet produces a static magnetic field, or B0 field, over the imaging volume. Similarly, gradient coils within the MR imaging system are employed to quickly switch into effect magnetic gradients along mutually orthogonal x, y, z coordinates in the static B0 field during selected portions of an MR imaging data acquisition cycle. Meanwhile, an RF coil produces RF magnetic field pulses, referred to as a B1 field, perpendicular to the B0 field, within the imaging volume to excite the nuclei. The nuclei are thereby excited to precess about an axis at a resonant RF frequency. As the precession occurs into the transverse plane, the transverse component of magnetization is magnetically coupled to some external circuitry, typically a receiver. These transmitter and receiver coupling mechanisms both are called RF coils.
RF antennas or coils are tuned and resonate in a frequency band defined by the Larmor frequency and the presence of a gradient field. The filling factor for RF coils is defined as the volume of sensitivity for a given RF coil. In general, the RF coil should be completely filled by the subject, as to eliminate unwanted noise sensitivity from the larger volume. This filling factor is very important. A transmitter coil or body coil, is designed to be uniformly sensitive over an entire field of view (FOV) as defined by the system. This design provides flexibility for imaging large or small volumes. However, the filling factor for such a coil is very large and, therefore, the potential signal to noise ratio (SNR) for a given scan is limited.
A design requirement for the RF transmitter coil is that it be extremely uniform over the entire imaging volume. This is necessary in order to generate a constant flip angle, tipping the B1 magnetization equally over the entire volume. The theory of reciprocity tells us that the B1 sensitivity for a transmit coil will be equal to its B1 receive sensitivity. Tradeoffs must be made between uniform B1 sensitivity and higher SNR. These tradeoffs are clearly seen when comparing the RF body coil to phased array received-only surface coils.
It is also desirable to have a coil permitting quadrature excitation and detection, or the ability to generate and receive circularly polarized fields. The most effective way to couple energy to or from the nuclei is by using fields that oscillate at the Larmor frequency. If these fields rotate in the same orientation as the magnetization, it maximizes energy coupling between the nuclei and the RF coils. A linearly polarized B1 can be decomposed into two counter-rotating or circularly polarized components where one half of the power rotates clockwise and the other one half of the power rotates counter-clockwise. Both have a magnitude of xc2xd B1. Only one of these components rotates in the same direction as the magnetization and, therefore, half of the power is wasted in a linear polarized field. By using quadrature excitation and detection, power is regained and signal increases by a factor of 2. A quadrature coil needs to split power equally into two channels, one delayed by 90xc2x0 with respect to the other. These two channels are then connected to the two inputs of the RF coil to produce two fields that are perpendicular to each other.
RF coil tradeoffs between signal uniformity and high SNR have led to separate designs of coils, one for transmitting and one for receiving the B1 magnetization. A surface coil is particularly useful to obtain images from tissues close to the surface of the patient. Surface coils provide higher SNR than whole-body coils because of smaller sensitive regions, leading to a decrease in noise coupled from the sample. The FOV of these coils is generally much smaller than volume coils. Therein lies the tradeoff. Phased array coils solve this problem by using multiple receiver channels to provide a larger region of sensitivity. Each of these coils is connected directly to an independent preamplifier and receiver chain. The magnitudes of these receivers are generally combined to form an image with extremely high SNR. The elimination of mutual inductance between independent coils must be achieved in order to provide uncorrelated noise and signal volumes for independent receiver coils.
In order to transmit and receive RF signals that optimize design tradeoffs between a volume transmit coil, with a high level of homogeneity, and small surface coils, able to achieve the highest SNR, separation of the RF transmit coil and the RF receiver coils is common practice. Historically, the invention of the birdcage RF volume coil has become a standard for homogenous transmit. The RF body coil is generally as large as 60 cm. in diameter and 50 to 70 cm. in length. System circuitry switches between the RF body coil and the various receiver coils. These receiver coils are arranged about the geometry of interest by using as many as 2 to 32 independent coils. The maximum number of receivers, within the data acquisition system, determines the maximum number of coil elements that the system is able to receive at any given time.
RF surface coils usually have inhomogeneous transmit fields. Their signal intensity decays as the distance from the center of the coil increases. When transmitting and receiving using surface coils, the field decays faster due to the addition of poor signal sensitivity from a surface coil during transmit and receive. There is a strong need to optimize and maximize the SNR from surface coils while maintaining the high homogenous transmit fields. A unique solution for achieving volume transmit field and optimizing independent receiver coils to improved SNR is, accordingly, an object of the present invention.
When scanning large patients, there is not always additional space to place surface coils around the anatomy. In these cases, a single volume coil must be used for both transmitting and receiving. A single transmit/receive surface array is desirable to transmit a uniform RF field, and receive as a phased array. This would improve SNR for large volume application. This would also decrease noise volumes during the independent coil reception, improve sensitivity near the coils, and increase the overall SNR, further objects of the present invention.
According to the present invention, a body coil is modeled after a birdcage coil and based on a ladder network design. A fixed number of single loop coils (N) are equally spaced around a cylinder. These coils must be driven 360xc2x0/N out of phase with respect to each other azimuthally. Each phase must increase 360xc2x0/N as azimuthally in order for single loop coil currents to mimic currents commonly seen in the quadrature birdcage coil. A switching circuit is necessary to change the transmit coil configuration and the N channel for phased array reception. This type of coil eliminates the otherwise necessary need to decouple the transmit coil from the receiver coils, but preserves the SNR benefit of having multiple receiver coils. This type of coil will improve SNR over ordinary volume coils and may be necessary imaging large patients where space around the patient is at a minimum.